In an optical pickup apparatus used for a magneto-optic storage device for reproducing information recorded in a magneto-optic recording medium, for example, a magneto-optic disk, a linear polarized laser beam is irradiated on a recording surface of the magneto-optic disk, and the information recorded as a magnetization direction on the recording surface is converted to a rotation of the polarization plane by the electrooptic Kerr effect or the Faraday effect, which are interactions between light and magnetization, and then detected.
Such an optical pickup apparatus for a magneto-optic storage device differs from an optical pickup apparatus which reproduces information from a compact disk (CD) such as an optical disk used only for reproduction, a write-once optical disk, a phase change type optical disk, etc., that is, an optical pickup apparatus for an optical recording and reproduction apparatus which detects the change of the amount of reflection light based on the presence/absence of pits.
In an optical system for a CD, as seen in a laser coupler, a reduction of size has been realized. However, in an optical system for an MO, it is technically difficult how to design that the reflection film of a beam splitter for splitting the MO signal can be used in the converged light, and thus the reduction of size as in the optical system for a CD has not yet been realized.
On the other hand, a polarization beam splitter has been known as an optical element which can exhibit desired characteristics even if it is placed in converged light, but it has been thought to be difficult to separate a MO signal using a polarization beam splitter. Nevertheless, several methods mentioned below have been known.
A conventional optical pickup apparatus for a magneto-optic storage device is constituted by for example a semiconductor laser; a collimate lens which collimates the light emitted from this semiconductor laser to a parallel beam; an objective lens which converges the emitted light collimated to the parallel beam by this collimate lens and irradiates the same on the recording surface of the magneto-optic disk; a polarization beam splitter which is arranged between the semiconductor laser and the objective lens and separates a part of the reflection light reflected at the recording surface of the magneto-optic disk (hereinafter referred to as the first polarization beam splitter); a light detector which detects a part of the reflection light split by this first polarization beam splitter; a photoelectric conversion element which detects the level of the light transmitted through this light detector (hereinafter referred to as a detector), etc. The emitted light from the semiconductor laser is converged and irradiated on the recording surface of the magneto-optic disk, a part of the reflection light reflected at this recording surface is split, and the level of the component having a predetermined polarization plane in a part of the split reflection light, that is, a predetermined light vibration plane is detected, and the information recorded in the magneto-optic disk is detected from that component.
As the above-mentioned light detector, a "sheet analyzer" has been generally used, but a sheet analyzer has a low "extinction rate", and therefore has a problem in that the S/N (signal to noise ratio) is low. This type of optical pickup apparatus for a magneto-optic storage device is not used much at all at present.
Therefore, an optical pickup apparatus for a magneto-optic storage device using a 1/2-wavelength plate and a second polarization beam splitter in place of the above-described light detector has been proposed. In this optical pickup apparatus for a magneto-optic storage device, the components of the reflection light split by the first polarization beam splitter, that is, the so-called P wave component, i.e., the component having the same plane of vibration as that of the emitted light from the laser, and the S wave component, which has a vibration direction orthogonal to this P wave component, are split by the 1/2-wavelength plate and the second polarization beam splitter in place of the light detector, the levels of the split orthogonal S wave component and P wave component are detected by the two detectors, a level difference between them is detected at a differential amplifier, and the recorded information is detected by the differentiation method.
In an optical pickup apparatus for a magneto-optic storage device using a 1/2-wavelength plate and a second polarization beam splitter in place of a light detector, since the S wave component of the laser beam split from the reflection light at the first polarization beam splitter is a magneto-optic signal (hereinafter referred to as an MO signal), the reflectance with respect to the S wave of the first polarization beam splitter is desirably set to 100 percent, but the reflectance with respect to the P wave component is still set so that the amount of light incident upon the detector and a shot noise of the detector or a noise due to the birefringence of the disk substrate or the like are well balanced, and therefore there is a problem in that the coupling efficiency is low in comparison with an optical pickup apparatus used only for reproduction etc. Also, the laser beam split at the first polarization beam splitter is split into two components by the second polarization beam splitter, and therefore it is necessary to establish the light path for it. The dimensions for the establishment of the light path are large, and thus the reduction of size of the optical pickup apparatus is difficult. Further, the coating for the polarization-splitting of the polarization beam splitter is difficult. For example, the coating technique for reducing the phase difference with respect to the P wave and S wave is very difficult, and therefore generally it is necessary to arrange the first polarization beam splitter in the parallel beam, and a collimate lens is always necessary.
Further, there has also been known an optical pickup apparatus for a magneto-optic storage device not using a 1/2-wavelength plate and a second polarization beam splitter, but using a Wollaston prism in place of the above-described light detector. However, this optical pickup apparatus for a magneto-optic storage device also meets with a similar problem to the one which occurred in the optical pickup apparatus for a magneto-optic storage device using a 1/2-wavelength plate and a second polarization beam splitter.